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Community and Q&A

Inadequate Cooling Problem

tundracycle | Posted in General Questions on

One of our bedrooms is not cooling well during summer. Even with the other registers in other rooms & hallway for this zone closed this room still does not cool well and other rooms still get extremely cold. Builder & HVAC contractor modified the room supply register that was doing a 180° turn under a window seat so that it was a 90° straight up from the floor but that only helped marginally.  Now they want to extend a new duct from the main duct in the hallway in to this room.

I have two concerns:

– The air from the new vent (green below), even cool/cold air, will largely short circuit from the new vent directly to the return without providing much cooling to the room.

– With more air to the return from this new vent the other two vents (room & bathroom in red below) will not provide as much airflow when the bedroom door is closed at night which is when cooling is most needed.

The existing vents: Red supply vent in lower wall of bathroom, red supply vent in floor of bedroom and blue return vent in upper wall of bedroom.

New vent: Green supply vent in floor.

The house was completed in Jun 2020. This is one of six HVAC zones and serves 2 UL bedrooms, 1 UL office and UL hallway.



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  1. tundracycle | | #1

    A couple of photos if they help to see the room, existing return and existing bedroom supply. Not on the supply that it was modified so no longer uses the portion in the window seat w/ the white register cover but now comes up from the floor.
    The original plan was to extend the new duct over to the window seat as well which is why the wood floor has not yet been fixed. Doing so will apparently be difficult so they are only going about halfway across the room instead.

  2. user-2310254 | | #2

    That's a lot of zones. How many systems? How long does the bedroom system run typically?

  3. tundracycle | | #3

    Two systems w/ 3 zones each. System 1: LL, ML North (kitchen, scullery, mudroom), UL Bedrooms (2 beds + baths, office, hallway). System 2: ML South (living rm & dining rm), UL Master, Loft. (It's a large house. If it were up to me it'd be smaller and have exterior insulation and ...)

    Typical run time varies. This room is our son and DIL's room when they are here and is otherwise a guest room. This past summer it was cooled 24x7 as they were living and working here compliments of Covid. Currently this room and the other guest room are unoccupied so dampers are closed (which realistically still allows 10-20% airflow?) resulting in this zone actively heating the hallway and my wife's office.

    Without Covid I'd guess that each guest room will be occupied maybe 150-200 days per year and mostly only heated/cooled at night.

  4. joshdurston | | #4

    It would be worth measuring the air volume to confirm you have low airflow. If the airflow is fine then it’s a unanticipated load. If it’s a load issue you may need to address the load directly. Otherwise you’ll need to fix the airflow.
    Is the window a western exposure?
    Measure the pressure drop across the door when it’s closed with a manometer to confirm you have a return air flow path.

  5. tundracycle | | #5

    Yes, the window faces west and given its size and shape I'm guessing accounts for a fair bit of heat during the day. The bigger cooling issue is at night though.

    We've measured the airflow at the two current supply registers and HVAC folks say that it's within range (I'll have to find my notes for exact numbers). The load at night is predominately 2 people who are both fit so about 90lbs & 150lbs. Maybe laptops & phones charging. No other electronics or heat production that I can think of.

    Good idea about the manometer w/ the door closed. I'll do that later today. Under the door as a return path is going to be minimal as bedroom and office doors are cut w/ minimal clearance (1/4 - 3/8") to help reduce noise transmission so the return path is heavily reliant on the return duct.

  6. woodguyatl | | #6

    Pure speculation here since I don’t have numbers, but this seems like a return problem, not a supply problem assuming the system has been balanced well on the supply side. Short circuiting won’t be an issue with the new vent, but if, as the contractor said, the current supply is good, won’t help much.

    1. user-6623302 | | #7

      My guess would be the same. Return air issues

  7. tundracycle | | #8

    "Short circuiting won’t be an issue with the new vent,"

    Can you help me understand this. Some air will be flowing from the two existing vents eastward across the room towards the return? How is air from the new vent going to flow against that flow? Either westward and then back eastward or northward and then eastward to the return?


  8. tundracycle | | #9

    Bumping this up. I'd really like to understand from someone how short circuiting from the new vent (green) to the return (blue) would not happen.


    1. Expert Member
      Michael Maines | | #10

      Some short-circuiting will happen but conditioned air spreads out pretty quickly, and you can use vanes on the supply register to direct the air away from the return. If you don't have enough total return air grill area, the return in that room may be pulling more than ideal.

      1. tundracycle | | #11

        Thanks Michael. I'm still struggling with it. Given its location I just can see how air from this register would go against the flow of the room to provide any benefit.

  9. tundracycle | | #12

    Update on this. We added a supply along the west wall. It is just south of the original supply (in red on the drawing) and is a 6" round hard duct from the main trunk. The original supply was moved north a bit.

    This helped the temp problem somewhat but didn't fix it. CO2 levels are still higher than they should be. The next step is likely trying to see if it's a return problem.

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